Vibration is often a problem in rotating machinery wherein a shaft rotates in a set of bearings. Vibration damping devices including an oil film in an annular space between the outer race of a bearing and the housing have been utilized. When the shaft moves off the bearing axis, the oil film exerts a damping force on the shaft. These damping devices are known as "squeeze film dampers."
In certain rotating machinery, such as aircraft engines, it is customary to utilize a dual shaft configuration wherein a first shaft rotates about an axis of rotation at one angular speed, and a second shaft concentric with the first shaft rotates about the same axis of rotation at a second angular speed. The two shafts are mounted by means of bearings to the housing, or frame of the machinery, so that they can rotate independently. Each of the shafts in the dual shaft assembly is subject to vibration. In some cases, the rotating shafts can become unstable, resulting in damage to the machine.
Squeeze film dampers have been proposed for use between the inner and outer shafts in a dual shaft configuration. The intershaft squeeze film damper suppresses relative movements of the two shafts, thereby reducing vibration. In the past, the intershaft squeeze film damper has been constructed as an annular oil film associated with either the inner race or the outer race of the intershaft bearing. The annular space which contains the damping oil film is defined by the surfaces of two concentric rings. When the intershaft squeeze film damper is associated with the inner race of the intershaft bearing, the damper, including the oil film, rotates at the speed of the inner shaft. When the squeeze film damper is associated with the outer race of the intershaft bearing, the oil film rotates at the speed of the outer shaft.
The intershaft squeeze film damper has been effective in reducing vibration levels at relatively low angular speeds of the two shafts. However, it has been found that the squeeze film damper produces instabilities at rotating speeds above a predetermined level. The range of speeds in which the instabilities occur is a function of the mass and stiffness of the two shafts. As a result, the usefulness of intershaft squeeze film dampers has been limited since instability often occurs at operating speeds of interest.
It is a general object of the present invention to provide an improved dual shaft rotating assembly.
It is another object of the present invention to provide a dual shaft rotating assembly having a stable intershaft squeeze film damper for attenuating vibration.
It is yet another object of the present invention to provide an intershaft squeeze film damper which is stable regardless of the rotating speeds of the two shafts.
It is still another object of the present invention to provide an intershaft squeeze film damper wherein the damping fluid does not rotate.